Current methods of directing the growth of cells are based on chemical gradients, polymer substrates, magnetic beads, or guidance channels. Methods involving polymer substrates and chemicals to direct neural growth are imprecise, static, and limited to two-dimensional applications. Magnetic bead steering is impractical due to the extensive preparation involved in attaching the beads to an axon, along with the added invasive risk. Electrically charged nerve guidance channels (U.S. Pat. No. 5,092,871 by Aebischer et al.) for repairing severed nerve ends have been described that place nerve ends in proximity to each other within a conduit of a guidance channel for promotion of nerve repair. This method is invasive and limited by the confines of the guidance channel.
Optical gradients have been used to interact with proteins, which are soft dielectrics. Optical tweezers (U.S. Pat. No. 5,079,169 by Chu et al and U.S. Pat. No. 5,245,466 by Burns et al) have been used to move and restrain biological particles, while optical stretchers (U.S. Pat. No. 6,067,859 by Kas & Guck) have been used to stretch and deform these dielectric materials. Current tools exploit optical gradients from a beam of light to manipulate the position of a small dielectric particle immersed in a fluid medium whose refractive index is smaller than that of the particle. These optical techniques have been generalized to enable manipulation of reflecting, absorbing, and low dielectric constant particles as well. Current optical trapping systems can manipulate single or multiple dielectric particle systems with one or more beam optical traps. Such systems have not been used to influence the growth or guide a cell.